Hello
Here is a graph that shows scan percentages for different setups, in the legend, SS stands for Signal Strength, and SR stands for Signature radius. Increasing SS and decreasing SR helps you avoid being probe scanned, SS is increased by fitting Signal Amps in low slots, or Signal Boosters in mid slots (with ECCM scrips for maximum effect)
The graph plots a Pod, and Shuttle, and a frigate with 3 different SS values
Conclusions
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Most of the benefits come from a larger Signal Strength rather than a smaller Sig Radius
This is seen from the Pod to Shuttle comparison, where the SR is the same for both. -
There is a limit where there is no point adding any more Signal Strength
This is seen from the bottom two plots where SR is the same but SS is much higher, but offers no more benefit in scan avoidance. -
A frigate can achieve the limit with 3 Signal Amplifiers or 1 or 2 ECCM scripted Signal Boosters
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From further tests, it became clear that the upper value for SS where adding any more is ~33
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Other poster in chat has stated that the useful upper value for SS is determined by a SS to SR ratio of about 1.1
So for example if SR = 100 then there is no point raising SS to any value higher than ~91
Note - the scanner ship was a Alpha, this is due to Omega status expiring in the past few days, and an outright refusal on my part to pay $20 (ÂŁ16) for another month of EVE. It should be noted that a skilled up Omega in any decent setup will be able to scan down anything, but this is about reducing the risks, not eliminating the risk.